Trialkoxysilanes, especially trimethoxysilane and triethoxysilane, are used in the production of silane coupling agents. One method of synthesis of trialkoxysilanes is directly from silicon and an alcohol. This method is known variously in the art as the Direct Synthesis, the Direct Reaction, the Direct Process or the Rochow Reaction. For trialkoxysilanes, it is most conveniently performed in slurry reactors.
In a slurry reactor for the Direct Synthesis of trialkoxysilanes, catalytically-activated silicon particles are maintained in suspension in a thermally stable, high boiling solvent and are made to react with an alcohol at an elevated temperature. This type of reaction is disclosed by Rochow in U.S. Pat. No. 3,641,077. The patent teaches preparation of trialkoxysilanes by directly reacting copper-silicon mass, suspended in a silicone oil, with alcohol at 250°-300° C. The copper-silicon mass contains about 10 weight percent copper and is prepared by heating copper and silicon above 1000° C. in a furnace in a stream of hydrogen gas. This method results in low yields of trialkoxysilanes.
U.S. Pat. No. 3,775,457 teaches the use of polyaromatic hydrocarbon oils as solvents in the Direct Synthesis of trialkoxysilanes from an alcohol and finely divided silicon metal activated with cuprous chloride catalyst. Although the use of cuprous chloride results in increased yield over that obtained using the sintered copper-silicon mass of U.S. Pat. No. 3,641,077, the use of cuprous chloride catalyst also results in the formation of HCl which, in turn, necessitates the use of costly corrosion resistant materials of construction for the reactor and its ancillary equipment. Further, the presence of chloride in the reactor and in the product stream reduces the yield of trialkoxy-silane by catalyzing the consecutive reaction of trialkoxysilane with the alcohol to yield tetra-alkoxysilanes.
Additionally, when methanol is a reactant, the HCl resulting from the use of the cuprous chloride catalyst will react with some of the methanol to produce methyl chloride and water. This loss of methanol to an undesirable side reaction makes the cuprous chloride catalyzed reaction inefficient. Moreover, water produced by this reaction can react with trialkoxysilanes and tetraalkoxysilanes to produce soluble and gelled siloxanes and further reduce the efficiency of the Direct Process. The presence of water in the reaction mixture can also inhibit the sustained conversion of silicon metal to desirable products at economically beneficial rates. Other patents, for example Japenese Kokai Tokkyo Koho 55-28928 (1980), 55-28929 (1980), 55-76891 (1980), 57-108094 (1982) and 62-96433 (1987), which disclose the use of cuprous chloride and cupric chloride and alkylated benzene solvents such as dodecylbenzene and tridecylbenzene, are subject to these same limitations. It is desirable to use the alkylated benzenes because they are less expensive and less hazardous to people and the environment than the polyaromatic hydrocarbon solvents of U.S. Pat. No. 3,775,457.
U.S. Pat. No. 4,727,173 discloses that the use of copper (II) hydroxide as catalyst avoids the limitations associated with cuprous chloride and provides a high selectivity to trialkoxysilanes. The preferred solvents are diphenyl ether, polyaromatic hydrocarbons like THERMINOL® 59, THERMINOL® 60 and THERMINOL® 66, and alkylated benzenes such as dodecylbenzene. However, when copper (II) hydroxide is used in combination with alkylated benzene solvents, such as dodecylbenzene, the Direct Synthesis of trialkoxysilanes becomes unstable after approximately 25-35 weight percent of the silicon has been reacted. When methanol is the alcohol reactant at temperatures above about 220° C., the trimethoxysilane content in the reaction product declines from approximately 90-95 weight percent to approximately 50-60 weight percent and recovers again to between 80-95 weight percent after about 60 percent silicon conversion. Simultaneous with this loss of selectivity is the enhanced formation of methane, water and dimethyl ether. Methane and dimethyl ether formation represent inefficient use of the methanol reagent. Problems attendant to the generation of water in the reaction mixture have been recited hereinabove.
Alcohol dehydration and dehydrogenation are especially troublesome problems when ethanol and other higher homologs are used in the Direct Synthesis. At some temperatures (>250° C.), alkenes and aldehydes, and not the desired trialkoxysilanes, are formed in significant amounts. Even when these are not the predominant products, their presence in the reaction mixture can result in the inhibition of further catalytic activity. At lower temperatures, (for example 220° C.) alcohol decomposition reactions are less prevalent, but the Direct Synthesis is impractically slow. Japanese Kokai Tokkyo Koho 55-2641 (1980) discloses the use of cyclic ethers to improve reactivity and selectivity to triethoxysilane when the Direct Synthesis is conducted in dodecylbenzene at these lower temperatures. Cyclic ethers such as dibenzo-18-crown-6 are quite expensive; others such as 12-crown-4 are also toxic.
U.S. Pat. No. 5,527,937 (European Patent application EP 0709388 A1) discloses a process for the Direct Synthesis of triethoxysilane and trimethoxysilane, wherein CuCl is the catalyst, tri- and tetra-toluenes and/or their alkyl substituted derivatives are the solvents and dimethylsilicone oils are antifoaming agents. The polyphenyl solvents of this process are expensive heat transfer fluids.
Foaming problems are also disclosed in Example 3 of U.S. Pat. No. 3,775,457 (German Patent 2,247,872). Foaming can lead to the partial or complete discharge of the reaction slurry from the reactor into the distillation and receiving vessels attached thereto. This is not only operationally inefficient with respect to raw material usage, but it also presents a difficult and time-consuming cleanup problem in laboratory, pilot and commercial scale reactions.
Thermal activation of slurries containing copper catalysts and silicon is disclosed in a number of patents, for example, U.S. Pat. Nos. 3,775,457 and 4,727,173. Use of hydrogen to activate silicon with copper for the Direct Reaction has been disclosed in U.S. Pat. Nos. 2,380,997; 2,473,260; 3,641,077; and 4,314,908. Hydrogen activation, as taught in these patents, is accomplished at temperatures above about 400° C. in fixed bed reactors, fluidized bed reactors or furnaces with silicon—copper catalyst mixtures containing more than 1.5 weight percent copper. No teaching is given regarding selectivity, reactivity and reaction stability of the silicon—copper masses in the slurry phase Direct Synthesis of trialkoxysilanes.
Suzuki, et al. (Bulletin of the Chemical Society of Japan, vol. 64 (1991) pp 3445-3447) disclosed that hydrogen activation of silicon—CuCl2 mixtures (2.5 wt % Cu) in a fixed bed at 260° C. afforded complete silicon conversion and high (89%) selectivity to trimethoxysilane in a fixed bed Direct Reaction with methanol. The duration of the induction period, the reaction rate and selectivity to trimethoxysilane were all very dependent on the temperature of hydrogen activation.
Thus, there continues to exist the need for a stable, highly selective and rapid Direct Synthesis of trialkoxysilanes which is conducted in cheaper, less hazardous solvents and yet avoids the above-mentioned deficiencies of copper chlorides and alkylated benzenes. In particular, there is a need for such a Direct Synthesis which eliminates or avoids the alcohol reduction and alcohol dehydration side reactions. These needs are addressed in a copending patent application filed on even date herewith and assigned to the assignee of the present application, bearing internal file number 89603 and entitled “Activation of Copper-Silicon Slurries for the Direct Synthesis of Trialkoxysilanes”.
There is also a need for a Direct trialkoxysilane Synthesis process in which foaming is controlled so that the reaction slurry is retained in the reactor. Moreover, the foam control method(s) must not have any deleterious effect(s) on the selectivity, rate and stability of the Direct Synthesis of trialkoxysilanes and must remain effective throughout the entire course of the reaction, especially when the solvent is used for more than one charge of silicon, when recycled solvent is used, or when the Direct Process is conducted continuously.